Retrospective Analysis of Clinical Presentation, Prognostic Factors and Outcome in 32 Dogs with Subcutaneous Mast Cell Tumours Treated with a Curative-Intent Approach.

Background: Subcutaneous mast cell tumours (ScMCTs) have been traditionally associated with a good prognosis, with low rates of recurrence and metastasis.

Objectives: This study aims to describe the clinical presentation, outcome, and prognostic factors in dogs diagnosed with ScMCTs and treated with a curative-intent approach.

Methods: Clinical and histopathological data were retrospectively collected from dogs diagnosed with ScMCTs after undergoing curative-intent surgery and complete staging between 2018 and 2023 in a single institution. Adjuvant and neoadjuvant therapies were allowed. The study's endpoints were the disease-free interval (DFI) and disease-specific survival time (DSST). Variables, including location, histopathological description, clinical stage, infiltrative behaviour, atypia, Darier's sign, surgical margins, mitotic count (MC) >4 in 10 high power fields (HPF), nodal status, and chemotherapy after surgery, were evaluated as potential influences on DFI and DSST.

Results: Thirty-two cases were included. Lymphadenectomies were performed in 18/32 (56.3%), and nodal metastases (early or overt) were documented in 12/32 (37.5%). The median follow-up was 405 days (range 79-1312). In 9/32 (28.1%), the disease progressed, and 7/32 (21.9%) died of ScMCT-related causes. The median DFI and DSST were not reached at 1312 days. The overall 1-year and 2-year survival rates were 80% and 70%, respectively. Patients presenting with Darier's sign, MC >4 in 10 HPF, and those who received chemotherapy had a higher risk of dying from the disease (hazard ratios of 14.9, 5.8 and 8.4, respectively).

Conclusions: Our results suggest that despite the overall good long-term prognosis of ScMCTs, they may exhibit a higher metastatic rate at presentation than previously reported. Additionally, patients with Darier's sign or a high mitotic count may be associated with a poorer prognosis.